A computer network is composed of a set of nodes and a set of links that connect one node to another. For instance, a computer network may be composed of a set of routers while the set of links may be cables between the routers. When a first node in the network sends a message to a second node in the network, the message may pass through many links and many nodes. The links and nodes the message passes through while traveling from the first node to the second node is referred to as a path.
Packet-based computer networks increasingly utilize label switching protocols for traffic engineering and other purposes. In a label switching network, label switching routers (LSRs) use Multi-Protocol Label Switching (MPLS) signaling protocols to establish label switched paths (LSPs). The LSRs utilize MPLS protocols to receive MPLS label mappings from downstream LSRs and to advertise MPLS label mappings to upstream LSRs. When an LSR receives an MPLS packet from an upstream router, it switches the MPLS label according to the information in its forwarding table and forwards the packet to the appropriate downstream LSR.
An LSP can be used to transport one or more services to network devices. One example of such a service is a Virtual Private Network (VPN) service. For example, Internet Protocol (IP) multicast traffic can travel from one Virtual Private Network (VPN) site to another within a Border Gateway Protocol (BGP)/MPLS IP VPN. Protocols and procedures for doing so are described in E. Rosen, “Multicast in MPLS/BGP IP VPNs,” Internet Engineering Task Force, RFC 6513, February 2012, the entire contents of which are incorporated by reference herein. One example procedure described in RFC 6513 for carrying MVPN data traffic through unicast tunnels is ingress replication. Ingress replication is a mechanism for distributing multicast traffic, which uses point-to-point (P2P) unicast tunnels between network devices to give an impression of multicast forwarding. An ingress network device of the P2P tunnels replicates packets received from a multicast source, and the ingress network device sends the replicated packets to a plurality of egress network devices via the respective P2P tunnels. The ingress network device (e.g., a provider edge (PE) router) can use an MPLS label to tag multicast VPN packets and send them through the same P2P LSPs that carry unicast VPN packets. The MPLS label used for tagging the VPN packets is downstream allocated and advertised by the egress PE router. One example use of ingress replication provider tunnels is a routing instance type of forwarding that uses ingress replication provider tunnels to carry IP multicast data between routers through an MPLS cloud, using MBGP (or Next Gen) MVPN. Ingress replication provider tunnels may be used for sending multicast traffic for providing various services, such as Internet Protocol Television (IPTV), desktop conferences, corporate broadcasts, music and video web casts, and other forms of multimedia content.
Links and nodes in a computer network may fail without warning. Consequently, one of the tunnels that the ingress network device was using to communicate with an egress network device may stop functioning. To ensure that the ingress network device and the egress network device do not send messages on a tunnel that is not functioning, the devices may periodically verify the status of the tunnel's path by sending a message through the path and receiving a response through the path. One mechanism for verifying connectivity along a tunnel such as an LSP, referred to as “LSP ping,” is described by RFC 4379, K. Kompella, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures”, RFC 4379, February 2006, hereby incorporated herein by reference. In general, LSP ping utilizes a form of MPLS “echo requests” and “echo replies” for detecting MPLS data plane failures and for verifying the MPLS LSP data plane against the control plane. Another mechanism for verifying connectivity along an LSP is Bidirectional Forwarding Detection (BFD). BFD is sometimes used on a per-service basis for those services transported by an LSP, which requires a separate BFD session for each service.